xref: /external/webp/src/dec/frame.c

// Copyright 2010 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// Frame-reconstruction function. Memory allocation.
//
// Author: Skal (pascal.massimino (at) gmail.com)

#include <stdlib.h>
#include "./vp8i.h"
#include "../utils/utils.h"

#define ALIGN_MASK (32 - 1)

static void ReconstructRow(const VP8Decoder* const dec,
                           const VP8ThreadContext* ctx);  // TODO(skal): remove

//------------------------------------------------------------------------------
// Filtering

// kFilterExtraRows[] = How many extra lines are needed on the MB boundary
// for caching, given a filtering level.
// Simple filter:  up to 2 luma samples are read and 1 is written.
// Complex filter: up to 4 luma samples are read and 3 are written. Same for
//                 U/V, so it's 8 samples total (because of the 2x upsampling).
static const uint8_t kFilterExtraRows[3] = { 0, 2, 8 };

static void DoFilter(const VP8Decoder* const dec, int mb_x, int mb_y) {
  const VP8ThreadContext* const ctx = &dec->thread_ctx_;
  const int cache_id = ctx->id_;
  const int y_bps = dec->cache_y_stride_;
  const VP8FInfo* const f_info = ctx->f_info_ + mb_x;
  uint8_t* const y_dst = dec->cache_y_ + cache_id * 16 * y_bps + mb_x * 16;
  const int ilevel = f_info->f_ilevel_;
  const int limit = f_info->f_limit_;
  if (limit == 0) {
    return;
  }
  assert(limit >= 3);
  if (dec->filter_type_ == 1) {   // simple
    if (mb_x > 0) {
      VP8SimpleHFilter16(y_dst, y_bps, limit + 4);
    }
    if (f_info->f_inner_) {
      VP8SimpleHFilter16i(y_dst, y_bps, limit);
    }
    if (mb_y > 0) {
      VP8SimpleVFilter16(y_dst, y_bps, limit + 4);
    }
    if (f_info->f_inner_) {
      VP8SimpleVFilter16i(y_dst, y_bps, limit);
    }
  } else {    // complex
    const int uv_bps = dec->cache_uv_stride_;
    uint8_t* const u_dst = dec->cache_u_ + cache_id * 8 * uv_bps + mb_x * 8;
    uint8_t* const v_dst = dec->cache_v_ + cache_id * 8 * uv_bps + mb_x * 8;
    const int hev_thresh = f_info->hev_thresh_;
    if (mb_x > 0) {
      VP8HFilter16(y_dst, y_bps, limit + 4, ilevel, hev_thresh);
      VP8HFilter8(u_dst, v_dst, uv_bps, limit + 4, ilevel, hev_thresh);
    }
    if (f_info->f_inner_) {
      VP8HFilter16i(y_dst, y_bps, limit, ilevel, hev_thresh);
      VP8HFilter8i(u_dst, v_dst, uv_bps, limit, ilevel, hev_thresh);
    }
    if (mb_y > 0) {
      VP8VFilter16(y_dst, y_bps, limit + 4, ilevel, hev_thresh);
      VP8VFilter8(u_dst, v_dst, uv_bps, limit + 4, ilevel, hev_thresh);
    }
    if (f_info->f_inner_) {
      VP8VFilter16i(y_dst, y_bps, limit, ilevel, hev_thresh);
      VP8VFilter8i(u_dst, v_dst, uv_bps, limit, ilevel, hev_thresh);
    }
  }
}

// Filter the decoded macroblock row (if needed)
static void FilterRow(const VP8Decoder* const dec) {
  int mb_x;
  const int mb_y = dec->thread_ctx_.mb_y_;
  assert(dec->thread_ctx_.filter_row_);
  for (mb_x = dec->tl_mb_x_; mb_x < dec->br_mb_x_; ++mb_x) {
    DoFilter(dec, mb_x, mb_y);
  }
}

//------------------------------------------------------------------------------
// Precompute the filtering strength for each segment and each i4x4/i16x16 mode.

static void PrecomputeFilterStrengths(VP8Decoder* const dec) {
  if (dec->filter_type_ > 0) {
    int s;
    const VP8FilterHeader* const hdr = &dec->filter_hdr_;
    for (s = 0; s < NUM_MB_SEGMENTS; ++s) {
      int i4x4;
      // First, compute the initial level
      int base_level;
      if (dec->segment_hdr_.use_segment_) {
        base_level = dec->segment_hdr_.filter_strength_[s];
        if (!dec->segment_hdr_.absolute_delta_) {
          base_level += hdr->level_;
        }
      } else {
        base_level = hdr->level_;
      }
      for (i4x4 = 0; i4x4 <= 1; ++i4x4) {
        VP8FInfo* const info = &dec->fstrengths_[s][i4x4];
        int level = base_level;
        if (hdr->use_lf_delta_) {
          // TODO(skal): only CURRENT is handled for now.
          level += hdr->ref_lf_delta_[0];
          if (i4x4) {
            level += hdr->mode_lf_delta_[0];
          }
        }
        level = (level < 0) ? 0 : (level > 63) ? 63 : level;
        if (level > 0) {
          int ilevel = level;
          if (hdr->sharpness_ > 0) {
            if (hdr->sharpness_ > 4) {
              ilevel >>= 2;
            } else {
              ilevel >>= 1;
            }
            if (ilevel > 9 - hdr->sharpness_) {
              ilevel = 9 - hdr->sharpness_;
            }
          }
          if (ilevel < 1) ilevel = 1;
          info->f_ilevel_ = ilevel;
          info->f_limit_ = 2 * level + ilevel;
          info->hev_thresh_ = (level >= 40) ? 2 : (level >= 15) ? 1 : 0;
        } else {
          info->f_limit_ = 0;  // no filtering
        }
        info->f_inner_ = i4x4;
      }
    }
  }
}

//------------------------------------------------------------------------------
// Dithering

#define DITHER_AMP_TAB_SIZE 12
static const int kQuantToDitherAmp[DITHER_AMP_TAB_SIZE] = {
  // roughly, it's dqm->uv_mat_[1]
  8, 7, 6, 4, 4, 2, 2, 2, 1, 1, 1, 1
};

void VP8InitDithering(const WebPDecoderOptions* const options,
                      VP8Decoder* const dec) {
  assert(dec != NULL);
  if (options != NULL) {
    const int d = options->dithering_strength;
    const int max_amp = (1 << VP8_RANDOM_DITHER_FIX) - 1;
    const int f = (d < 0) ? 0 : (d > 100) ? max_amp : (d * max_amp / 100);
    if (f > 0) {
      int s;
      int all_amp = 0;
      for (s = 0; s < NUM_MB_SEGMENTS; ++s) {
        VP8QuantMatrix* const dqm = &dec->dqm_[s];
        if (dqm->uv_quant_ < DITHER_AMP_TAB_SIZE) {
          // TODO(skal): should we specially dither more for uv_quant_ < 0?
          const int idx = (dqm->uv_quant_ < 0) ? 0 : dqm->uv_quant_;
          dqm->dither_ = (f * kQuantToDitherAmp[idx]) >> 3;
        }
        all_amp |= dqm->dither_;
      }
      if (all_amp != 0) {
        VP8InitRandom(&dec->dithering_rg_, 1.0f);
        dec->dither_ = 1;
      }
    }
#if WEBP_DECODER_ABI_VERSION > 0x0204
    // potentially allow alpha dithering
    dec->alpha_dithering_ = options->alpha_dithering_strength;
    if (dec->alpha_dithering_ > 100) {
      dec->alpha_dithering_ = 100;
    } else if (dec->alpha_dithering_ < 0) {
      dec->alpha_dithering_ = 0;
    }
#endif
  }
}

// minimal amp that will provide a non-zero dithering effect
#define MIN_DITHER_AMP 4
#define DITHER_DESCALE 4
#define DITHER_DESCALE_ROUNDER (1 << (DITHER_DESCALE - 1))
#define DITHER_AMP_BITS 8
#define DITHER_AMP_CENTER (1 << DITHER_AMP_BITS)

static void Dither8x8(VP8Random* const rg, uint8_t* dst, int bps, int amp) {
  int i, j;
  for (j = 0; j < 8; ++j) {
    for (i = 0; i < 8; ++i) {
      // TODO: could be made faster with SSE2
      const int bits =
          VP8RandomBits2(rg, DITHER_AMP_BITS + 1, amp) - DITHER_AMP_CENTER;
      // Convert to range: [-2,2] for dither=50, [-4,4] for dither=100
      const int delta = (bits + DITHER_DESCALE_ROUNDER) >> DITHER_DESCALE;
      const int v = (int)dst[i] + delta;
      dst[i] = (v < 0) ? 0 : (v > 255) ? 255u : (uint8_t)v;
    }
    dst += bps;
  }
}

static void DitherRow(VP8Decoder* const dec) {
  int mb_x;
  assert(dec->dither_);
  for (mb_x = dec->tl_mb_x_; mb_x < dec->br_mb_x_; ++mb_x) {
    const VP8ThreadContext* const ctx = &dec->thread_ctx_;
    const VP8MBData* const data = ctx->mb_data_ + mb_x;
    const int cache_id = ctx->id_;
    const int uv_bps = dec->cache_uv_stride_;
    if (data->dither_ >= MIN_DITHER_AMP) {
      uint8_t* const u_dst = dec->cache_u_ + cache_id * 8 * uv_bps + mb_x * 8;
      uint8_t* const v_dst = dec->cache_v_ + cache_id * 8 * uv_bps + mb_x * 8;
      Dither8x8(&dec->dithering_rg_, u_dst, uv_bps, data->dither_);
      Dither8x8(&dec->dithering_rg_, v_dst, uv_bps, data->dither_);
    }
  }
}

//------------------------------------------------------------------------------
// This function is called after a row of macroblocks is finished decoding.
// It also takes into account the following restrictions:
//  * In case of in-loop filtering, we must hold off sending some of the bottom
//    pixels as they are yet unfiltered. They will be when the next macroblock
//    row is decoded. Meanwhile, we must preserve them by rotating them in the
//    cache area. This doesn't hold for the very bottom row of the uncropped
//    picture of course.
//  * we must clip the remaining pixels against the cropping area. The VP8Io
//    struct must have the following fields set correctly before calling put():

#define MACROBLOCK_VPOS(mb_y)  ((mb_y) * 16)    // vertical position of a MB

// Finalize and transmit a complete row. Return false in case of user-abort.
static int FinishRow(VP8Decoder* const dec, VP8Io* const io) {
  int ok = 1;
  const VP8ThreadContext* const ctx = &dec->thread_ctx_;
  const int cache_id = ctx->id_;
  const int extra_y_rows = kFilterExtraRows[dec->filter_type_];
  const int ysize = extra_y_rows * dec->cache_y_stride_;
  const int uvsize = (extra_y_rows / 2) * dec->cache_uv_stride_;
  const int y_offset = cache_id * 16 * dec->cache_y_stride_;
  const int uv_offset = cache_id * 8 * dec->cache_uv_stride_;
  uint8_t* const ydst = dec->cache_y_ - ysize + y_offset;
  uint8_t* const udst = dec->cache_u_ - uvsize + uv_offset;
  uint8_t* const vdst = dec->cache_v_ - uvsize + uv_offset;
  const int mb_y = ctx->mb_y_;
  const int is_first_row = (mb_y == 0);
  const int is_last_row = (mb_y >= dec->br_mb_y_ - 1);

  if (dec->mt_method_ == 2) {
    ReconstructRow(dec, ctx);
  }

  if (ctx->filter_row_) {
    FilterRow(dec);
  }

  if (dec->dither_) {
    DitherRow(dec);
  }

  if (io->put != NULL) {
    int y_start = MACROBLOCK_VPOS(mb_y);
    int y_end = MACROBLOCK_VPOS(mb_y + 1);
    if (!is_first_row) {
      y_start -= extra_y_rows;
      io->y = ydst;
      io->u = udst;
      io->v = vdst;
    } else {
      io->y = dec->cache_y_ + y_offset;
      io->u = dec->cache_u_ + uv_offset;
      io->v = dec->cache_v_ + uv_offset;
    }

    if (!is_last_row) {
      y_end -= extra_y_rows;
    }
    if (y_end > io->crop_bottom) {
      y_end = io->crop_bottom;    // make sure we don't overflow on last row.
    }
    io->a = NULL;
    if (dec->alpha_data_ != NULL && y_start < y_end) {
      // TODO(skal): testing presence of alpha with dec->alpha_data_ is not a
      // good idea.
      io->a = VP8DecompressAlphaRows(dec, y_start, y_end - y_start);
      if (io->a == NULL) {
        return VP8SetError(dec, VP8_STATUS_BITSTREAM_ERROR,
                           "Could not decode alpha data.");
      }
    }
    if (y_start < io->crop_top) {
      const int delta_y = io->crop_top - y_start;
      y_start = io->crop_top;
      assert(!(delta_y & 1));
      io->y += dec->cache_y_stride_ * delta_y;
      io->u += dec->cache_uv_stride_ * (delta_y >> 1);
      io->v += dec->cache_uv_stride_ * (delta_y >> 1);
      if (io->a != NULL) {
        io->a += io->width * delta_y;
      }
    }
    if (y_start < y_end) {
      io->y += io->crop_left;
      io->u += io->crop_left >> 1;
      io->v += io->crop_left >> 1;
      if (io->a != NULL) {
        io->a += io->crop_left;
      }
      io->mb_y = y_start - io->crop_top;
      io->mb_w = io->crop_right - io->crop_left;
      io->mb_h = y_end - y_start;
      ok = io->put(io);
    }
  }
  // rotate top samples if needed
  if (cache_id + 1 == dec->num_caches_) {
    if (!is_last_row) {
      memcpy(dec->cache_y_ - ysize, ydst + 16 * dec->cache_y_stride_, ysize);
      memcpy(dec->cache_u_ - uvsize, udst + 8 * dec->cache_uv_stride_, uvsize);
      memcpy(dec->cache_v_ - uvsize, vdst + 8 * dec->cache_uv_stride_, uvsize);
    }
  }

  return ok;
}

#undef MACROBLOCK_VPOS

//------------------------------------------------------------------------------

int VP8ProcessRow(VP8Decoder* const dec, VP8Io* const io) {
  int ok = 1;
  VP8ThreadContext* const ctx = &dec->thread_ctx_;
  const int filter_row =
      (dec->filter_type_ > 0) &&
      (dec->mb_y_ >= dec->tl_mb_y_) && (dec->mb_y_ <= dec->br_mb_y_);
  if (dec->mt_method_ == 0) {
    // ctx->id_ and ctx->f_info_ are already set
    ctx->mb_y_ = dec->mb_y_;
    ctx->filter_row_ = filter_row;
    ReconstructRow(dec, ctx);
    ok = FinishRow(dec, io);
  } else {
    WebPWorker* const worker = &dec->worker_;
    // Finish previous job *before* updating context
    ok &= WebPGetWorkerInterface()->Sync(worker);
    assert(worker->status_ == OK);
    if (ok) {   // spawn a new deblocking/output job
      ctx->io_ = *io;
      ctx->id_ = dec->cache_id_;
      ctx->mb_y_ = dec->mb_y_;
      ctx->filter_row_ = filter_row;
      if (dec->mt_method_ == 2) {  // swap macroblock data
        VP8MBData* const tmp = ctx->mb_data_;
        ctx->mb_data_ = dec->mb_data_;
        dec->mb_data_ = tmp;
      } else {
        // perform reconstruction directly in main thread
        ReconstructRow(dec, ctx);
      }
      if (filter_row) {            // swap filter info
        VP8FInfo* const tmp = ctx->f_info_;
        ctx->f_info_ = dec->f_info_;
        dec->f_info_ = tmp;
      }
      // (reconstruct)+filter in parallel
      WebPGetWorkerInterface()->Launch(worker);
      if (++dec->cache_id_ == dec->num_caches_) {
        dec->cache_id_ = 0;
      }
    }
  }
  return ok;
}

//------------------------------------------------------------------------------
// Finish setting up the decoding parameter once user's setup() is called.

VP8StatusCode VP8EnterCritical(VP8Decoder* const dec, VP8Io* const io) {
  // Call setup() first. This may trigger additional decoding features on 'io'.
  // Note: Afterward, we must call teardown() no matter what.
  if (io->setup != NULL && !io->setup(io)) {
    VP8SetError(dec, VP8_STATUS_USER_ABORT, "Frame setup failed");
    return dec->status_;
  }

  // Disable filtering per user request
  if (io->bypass_filtering) {
    dec->filter_type_ = 0;
  }
  // TODO(skal): filter type / strength / sharpness forcing

  // Define the area where we can skip in-loop filtering, in case of cropping.
  //
  // 'Simple' filter reads two luma samples outside of the macroblock
  // and filters one. It doesn't filter the chroma samples. Hence, we can
  // avoid doing the in-loop filtering before crop_top/crop_left position.
  // For the 'Complex' filter, 3 samples are read and up to 3 are filtered.
  // Means: there's a dependency chain that goes all the way up to the
  // top-left corner of the picture (MB #0). We must filter all the previous
  // macroblocks.
  // TODO(skal): add an 'approximate_decoding' option, that won't produce
  // a 1:1 bit-exactness for complex filtering?
  {
    const int extra_pixels = kFilterExtraRows[dec->filter_type_];
    if (dec->filter_type_ == 2) {
      // For complex filter, we need to preserve the dependency chain.
      dec->tl_mb_x_ = 0;
      dec->tl_mb_y_ = 0;
    } else {
      // For simple filter, we can filter only the cropped region.
      // We include 'extra_pixels' on the other side of the boundary, since
      // vertical or horizontal filtering of the previous macroblock can
      // modify some abutting pixels.
      dec->tl_mb_x_ = (io->crop_left - extra_pixels) >> 4;
      dec->tl_mb_y_ = (io->crop_top - extra_pixels) >> 4;
      if (dec->tl_mb_x_ < 0) dec->tl_mb_x_ = 0;
      if (dec->tl_mb_y_ < 0) dec->tl_mb_y_ = 0;
    }
    // We need some 'extra' pixels on the right/bottom.
    dec->br_mb_y_ = (io->crop_bottom + 15 + extra_pixels) >> 4;
    dec->br_mb_x_ = (io->crop_right + 15 + extra_pixels) >> 4;
    if (dec->br_mb_x_ > dec->mb_w_) {
      dec->br_mb_x_ = dec->mb_w_;
    }
    if (dec->br_mb_y_ > dec->mb_h_) {
      dec->br_mb_y_ = dec->mb_h_;
    }
  }
  PrecomputeFilterStrengths(dec);
  return VP8_STATUS_OK;
}

int VP8ExitCritical(VP8Decoder* const dec, VP8Io* const io) {
  int ok = 1;
  if (dec->mt_method_ > 0) {
    ok = WebPGetWorkerInterface()->Sync(&dec->worker_);
  }

  if (io->teardown != NULL) {
    io->teardown(io);
  }
  return ok;
}

//------------------------------------------------------------------------------
// For multi-threaded decoding we need to use 3 rows of 16 pixels as delay line.
//
// Reason is: the deblocking filter cannot deblock the bottom horizontal edges
// immediately, and needs to wait for first few rows of the next macroblock to
// be decoded. Hence, deblocking is lagging behind by 4 or 8 pixels (depending
// on strength).
// With two threads, the vertical positions of the rows being decoded are:
// Decode:  [ 0..15][16..31][32..47][48..63][64..79][...
// Deblock:         [ 0..11][12..27][28..43][44..59][...
// If we use two threads and two caches of 16 pixels, the sequence would be:
// Decode:  [ 0..15][16..31][ 0..15!!][16..31][ 0..15][...
// Deblock:         [ 0..11][12..27!!][-4..11][12..27][...
// The problem occurs during row [12..15!!] that both the decoding and
// deblocking threads are writing simultaneously.
// With 3 cache lines, one get a safe write pattern:
// Decode:  [ 0..15][16..31][32..47][ 0..15][16..31][32..47][0..
// Deblock:         [ 0..11][12..27][28..43][-4..11][12..27][28...
// Note that multi-threaded output _without_ deblocking can make use of two
// cache lines of 16 pixels only, since there's no lagging behind. The decoding
// and output process have non-concurrent writing:
// Decode:  [ 0..15][16..31][ 0..15][16..31][...
// io->put:         [ 0..15][16..31][ 0..15][...

#define MT_CACHE_LINES 3
#define ST_CACHE_LINES 1   // 1 cache row only for single-threaded case

// Initialize multi/single-thread worker
static int InitThreadContext(VP8Decoder* const dec) {
  dec->cache_id_ = 0;
  if (dec->mt_method_ > 0) {
    WebPWorker* const worker = &dec->worker_;
    if (!WebPGetWorkerInterface()->Reset(worker)) {
      return VP8SetError(dec, VP8_STATUS_OUT_OF_MEMORY,
                         "thread initialization failed.");
    }
    worker->data1 = dec;
    worker->data2 = (void*)&dec->thread_ctx_.io_;
    worker->hook = (WebPWorkerHook)FinishRow;
    dec->num_caches_ =
      (dec->filter_type_ > 0) ? MT_CACHE_LINES : MT_CACHE_LINES - 1;
  } else {
    dec->num_caches_ = ST_CACHE_LINES;
  }
  return 1;
}

int VP8GetThreadMethod(const WebPDecoderOptions* const options,
                       const WebPHeaderStructure* const headers,
                       int width, int height) {
  if (options == NULL || options->use_threads == 0) {
    return 0;
  }
  (void)headers;
  (void)width;
  (void)height;
  assert(headers == NULL || !headers->is_lossless);
#if defined(WEBP_USE_THREAD)
  if (width < MIN_WIDTH_FOR_THREADS) return 0;
  // TODO(skal): tune the heuristic further
#if 0
  if (height < 2 * width) return 2;
#endif
  return 2;
#else   // !WEBP_USE_THREAD
  return 0;
#endif
}

#undef MT_CACHE_LINES
#undef ST_CACHE_LINES

//------------------------------------------------------------------------------
// Memory setup

static int AllocateMemory(VP8Decoder* const dec) {
  const int num_caches = dec->num_caches_;
  const int mb_w = dec->mb_w_;
  // Note: we use 'size_t' when there's no overflow risk, uint64_t otherwise.
  const size_t intra_pred_mode_size = 4 * mb_w * sizeof(uint8_t);
  const size_t top_size = sizeof(VP8TopSamples) * mb_w;
  const size_t mb_info_size = (mb_w + 1) * sizeof(VP8MB);
  const size_t f_info_size =
      (dec->filter_type_ > 0) ?
          mb_w * (dec->mt_method_ > 0 ? 2 : 1) * sizeof(VP8FInfo)
        : 0;
  const size_t yuv_size = YUV_SIZE * sizeof(*dec->yuv_b_);
  const size_t mb_data_size =
      (dec->mt_method_ == 2 ? 2 : 1) * mb_w * sizeof(*dec->mb_data_);
  const size_t cache_height = (16 * num_caches
                            + kFilterExtraRows[dec->filter_type_]) * 3 / 2;
  const size_t cache_size = top_size * cache_height;
  // alpha_size is the only one that scales as width x height.
  const uint64_t alpha_size = (dec->alpha_data_ != NULL) ?
      (uint64_t)dec->pic_hdr_.width_ * dec->pic_hdr_.height_ : 0ULL;
  const uint64_t needed = (uint64_t)intra_pred_mode_size
                        + top_size + mb_info_size + f_info_size
                        + yuv_size + mb_data_size
                        + cache_size + alpha_size + ALIGN_MASK;
  uint8_t* mem;

  if (needed != (size_t)needed) return 0;  // check for overflow
  if (needed > dec->mem_size_) {
    WebPSafeFree(dec->mem_);
    dec->mem_size_ = 0;
    dec->mem_ = WebPSafeMalloc(needed, sizeof(uint8_t));
    if (dec->mem_ == NULL) {
      return VP8SetError(dec, VP8_STATUS_OUT_OF_MEMORY,
                         "no memory during frame initialization.");
    }
    // down-cast is ok, thanks to WebPSafeAlloc() above.
    dec->mem_size_ = (size_t)needed;
  }

  mem = (uint8_t*)dec->mem_;
  dec->intra_t_ = (uint8_t*)mem;
  mem += intra_pred_mode_size;

  dec->yuv_t_ = (VP8TopSamples*)mem;
  mem += top_size;

  dec->mb_info_ = ((VP8MB*)mem) + 1;
  mem += mb_info_size;

  dec->f_info_ = f_info_size ? (VP8FInfo*)mem : NULL;
  mem += f_info_size;
  dec->thread_ctx_.id_ = 0;
  dec->thread_ctx_.f_info_ = dec->f_info_;
  if (dec->mt_method_ > 0) {
    // secondary cache line. The deblocking process need to make use of the
    // filtering strength from previous macroblock row, while the new ones
    // are being decoded in parallel. We'll just swap the pointers.
    dec->thread_ctx_.f_info_ += mb_w;
  }

  mem = (uint8_t*)((uintptr_t)(mem + ALIGN_MASK) & ~ALIGN_MASK);
  assert((yuv_size & ALIGN_MASK) == 0);
  dec->yuv_b_ = (uint8_t*)mem;
  mem += yuv_size;

  dec->mb_data_ = (VP8MBData*)mem;
  dec->thread_ctx_.mb_data_ = (VP8MBData*)mem;
  if (dec->mt_method_ == 2) {
    dec->thread_ctx_.mb_data_ += mb_w;
  }
  mem += mb_data_size;

  dec->cache_y_stride_ = 16 * mb_w;
  dec->cache_uv_stride_ = 8 * mb_w;
  {
    const int extra_rows = kFilterExtraRows[dec->filter_type_];
    const int extra_y = extra_rows * dec->cache_y_stride_;
    const int extra_uv = (extra_rows / 2) * dec->cache_uv_stride_;
    dec->cache_y_ = ((uint8_t*)mem) + extra_y;
    dec->cache_u_ = dec->cache_y_
                  + 16 * num_caches * dec->cache_y_stride_ + extra_uv;
    dec->cache_v_ = dec->cache_u_
                  + 8 * num_caches * dec->cache_uv_stride_ + extra_uv;
    dec->cache_id_ = 0;
  }
  mem += cache_size;

  // alpha plane
  dec->alpha_plane_ = alpha_size ? (uint8_t*)mem : NULL;
  mem += alpha_size;
  assert(mem <= (uint8_t*)dec->mem_ + dec->mem_size_);

  // note: left/top-info is initialized once for all.
  memset(dec->mb_info_ - 1, 0, mb_info_size);
  VP8InitScanline(dec);   // initialize left too.

  // initialize top
  memset(dec->intra_t_, B_DC_PRED, intra_pred_mode_size);

  return 1;
}

static void InitIo(VP8Decoder* const dec, VP8Io* io) {
  // prepare 'io'
  io->mb_y = 0;
  io->y = dec->cache_y_;
  io->u = dec->cache_u_;
  io->v = dec->cache_v_;
  io->y_stride = dec->cache_y_stride_;
  io->uv_stride = dec->cache_uv_stride_;
  io->a = NULL;
}

int VP8InitFrame(VP8Decoder* const dec, VP8Io* io) {
  if (!InitThreadContext(dec)) return 0;  // call first. Sets dec->num_caches_.
  if (!AllocateMemory(dec)) return 0;
  InitIo(dec, io);
  VP8DspInit();  // Init critical function pointers and look-up tables.
  return 1;
}

//------------------------------------------------------------------------------
// Main reconstruction function.

static const int kScan[16] = {
  0 +  0 * BPS,  4 +  0 * BPS, 8 +  0 * BPS, 12 +  0 * BPS,
  0 +  4 * BPS,  4 +  4 * BPS, 8 +  4 * BPS, 12 +  4 * BPS,
  0 +  8 * BPS,  4 +  8 * BPS, 8 +  8 * BPS, 12 +  8 * BPS,
  0 + 12 * BPS,  4 + 12 * BPS, 8 + 12 * BPS, 12 + 12 * BPS
};

static int CheckMode(int mb_x, int mb_y, int mode) {
  if (mode == B_DC_PRED) {
    if (mb_x == 0) {
      return (mb_y == 0) ? B_DC_PRED_NOTOPLEFT : B_DC_PRED_NOLEFT;
    } else {
      return (mb_y == 0) ? B_DC_PRED_NOTOP : B_DC_PRED;
    }
  }
  return mode;
}

static void Copy32b(uint8_t* dst, uint8_t* src) {
  memcpy(dst, src, 4);
}

static WEBP_INLINE void DoTransform(uint32_t bits, const int16_t* const src,
                                    uint8_t* const dst) {
  switch (bits >> 30) {
    case 3:
      VP8Transform(src, dst, 0);
      break;
    case 2:
      VP8TransformAC3(src, dst);
      break;
    case 1:
      VP8TransformDC(src, dst);
      break;
    default:
      break;
  }
}

static void DoUVTransform(uint32_t bits, const int16_t* const src,
                          uint8_t* const dst) {
  if (bits & 0xff) {    // any non-zero coeff at all?
    if (bits & 0xaa) {  // any non-zero AC coefficient?
      VP8TransformUV(src, dst);   // note we don't use the AC3 variant for U/V
    } else {
      VP8TransformDCUV(src, dst);
    }
  }
}

static void ReconstructRow(const VP8Decoder* const dec,
                           const VP8ThreadContext* ctx) {
  int j;
  int mb_x;
  const int mb_y = ctx->mb_y_;
  const int cache_id = ctx->id_;
  uint8_t* const y_dst = dec->yuv_b_ + Y_OFF;
  uint8_t* const u_dst = dec->yuv_b_ + U_OFF;
  uint8_t* const v_dst = dec->yuv_b_ + V_OFF;
  for (mb_x = 0; mb_x < dec->mb_w_; ++mb_x) {
    const VP8MBData* const block = ctx->mb_data_ + mb_x;

    // Rotate in the left samples from previously decoded block. We move four
    // pixels at a time for alignment reason, and because of in-loop filter.
    if (mb_x > 0) {
      for (j = -1; j < 16; ++j) {
        Copy32b(&y_dst[j * BPS - 4], &y_dst[j * BPS + 12]);
      }
      for (j = -1; j < 8; ++j) {
        Copy32b(&u_dst[j * BPS - 4], &u_dst[j * BPS + 4]);
        Copy32b(&v_dst[j * BPS - 4], &v_dst[j * BPS + 4]);
      }
    } else {
      for (j = 0; j < 16; ++j) {
        y_dst[j * BPS - 1] = 129;
      }
      for (j = 0; j < 8; ++j) {
        u_dst[j * BPS - 1] = 129;
        v_dst[j * BPS - 1] = 129;
      }
      // Init top-left sample on left column too
      if (mb_y > 0) {
        y_dst[-1 - BPS] = u_dst[-1 - BPS] = v_dst[-1 - BPS] = 129;
      }
    }
    {
      // bring top samples into the cache
      VP8TopSamples* const top_yuv = dec->yuv_t_ + mb_x;
      const int16_t* const coeffs = block->coeffs_;
      uint32_t bits = block->non_zero_y_;
      int n;

      if (mb_y > 0) {
        memcpy(y_dst - BPS, top_yuv[0].y, 16);
        memcpy(u_dst - BPS, top_yuv[0].u, 8);
        memcpy(v_dst - BPS, top_yuv[0].v, 8);
      } else if (mb_x == 0) {
        // we only need to do this init once at block (0,0).
        // Afterward, it remains valid for the whole topmost row.
        memset(y_dst - BPS - 1, 127, 16 + 4 + 1);
        memset(u_dst - BPS - 1, 127, 8 + 1);
        memset(v_dst - BPS - 1, 127, 8 + 1);
      }

      // predict and add residuals
      if (block->is_i4x4_) {   // 4x4
        uint32_t* const top_right = (uint32_t*)(y_dst - BPS + 16);

        if (mb_y > 0) {
          if (mb_x >= dec->mb_w_ - 1) {    // on rightmost border
            memset(top_right, top_yuv[0].y[15], sizeof(*top_right));
          } else {
            memcpy(top_right, top_yuv[1].y, sizeof(*top_right));
          }
        }
        // replicate the top-right pixels below
        top_right[BPS] = top_right[2 * BPS] = top_right[3 * BPS] = top_right[0];

        // predict and add residuals for all 4x4 blocks in turn.
        for (n = 0; n < 16; ++n, bits <<= 2) {
          uint8_t* const dst = y_dst + kScan[n];
          VP8PredLuma4[block->imodes_[n]](dst);
          DoTransform(bits, coeffs + n * 16, dst);
        }
      } else {    // 16x16
        const int pred_func = CheckMode(mb_x, mb_y,
                                        block->imodes_[0]);
        VP8PredLuma16[pred_func](y_dst);
        if (bits != 0) {
          for (n = 0; n < 16; ++n, bits <<= 2) {
            DoTransform(bits, coeffs + n * 16, y_dst + kScan[n]);
          }
        }
      }
      {
        // Chroma
        const uint32_t bits_uv = block->non_zero_uv_;
        const int pred_func = CheckMode(mb_x, mb_y, block->uvmode_);
        VP8PredChroma8[pred_func](u_dst);
        VP8PredChroma8[pred_func](v_dst);
        DoUVTransform(bits_uv >> 0, coeffs + 16 * 16, u_dst);
        DoUVTransform(bits_uv >> 8, coeffs + 20 * 16, v_dst);
      }

      // stash away top samples for next block
      if (mb_y < dec->mb_h_ - 1) {
        memcpy(top_yuv[0].y, y_dst + 15 * BPS, 16);
        memcpy(top_yuv[0].u, u_dst +  7 * BPS,  8);
        memcpy(top_yuv[0].v, v_dst +  7 * BPS,  8);
      }
    }
    // Transfer reconstructed samples from yuv_b_ cache to final destination.
    {
      const int y_offset = cache_id * 16 * dec->cache_y_stride_;
      const int uv_offset = cache_id * 8 * dec->cache_uv_stride_;
      uint8_t* const y_out = dec->cache_y_ + mb_x * 16 + y_offset;
      uint8_t* const u_out = dec->cache_u_ + mb_x * 8 + uv_offset;
      uint8_t* const v_out = dec->cache_v_ + mb_x * 8 + uv_offset;
      for (j = 0; j < 16; ++j) {
        memcpy(y_out + j * dec->cache_y_stride_, y_dst + j * BPS, 16);
      }
      for (j = 0; j < 8; ++j) {
        memcpy(u_out + j * dec->cache_uv_stride_, u_dst + j * BPS, 8);
        memcpy(v_out + j * dec->cache_uv_stride_, v_dst + j * BPS, 8);
      }
    }
  }
}

//------------------------------------------------------------------------------